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Fatigue growth of short cracks in Ti-17: experiments and simulations
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).ORCID iD: 0000-0001-6896-1834
2007 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 74, no 15, 2293-2310 p.Article in journal (Refereed) Published
Abstract [en]

The fatigue behaviour of through thickness short cracks was investigated in Ti-17. Experiments were performed on a symmetric four-point bend set-up. An initial through thickness crack was produced by cyclic compressive load on a sharp notch. The notch and part of the crack were removed leaving an approximately 50 mu m short crack. The short crack was subjected to fatigue loading in tension. The experiments were conducted in load control with constant force amplitude and mean values. Fatigue growth of the short cracks was monitored with direct current potential drop measurements. Fatigue growth continued at constant R-ratio into the long crack regime. It was found that linear elastic fracture mechanics (LEFM) was applicable if closure-free long crack growth data from constant K-1max test were used. Then, the standard Paris' relation provided an upper bound for the growth rates of both short and long crack.The short crack experiments were numerically reproduced in two ways by finite element computations. The first analysis type comprised all three phases of the experimental procedure: precracking, notch removal and fatigue growth. The second analysis type only reproduced the growth of short cracks during fatigue loading in tension. In both cases the material model was elastic-plastic with combined isotropic and kinematic hardening. The agreement between crack tip opening displacement range, cyclic J-integral and cyclic plastic zone at the crack tip with Delta K-1 verified that LEFM could be extended to the present short cracks in Ti-17. Also, the crack size limits described in the literature for LEFM with regards to plastic zone size hold for the present short cracks and cyclic softening material.

Place, publisher, year, edition, pages
2007. Vol. 74, no 15, 2293-2310 p.
Keyword [en]
short crack, fatigue crack growth, potential drop method, titanium alloy, crack closure
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-6423DOI: 10.1016/j.engfracmech.2006.11.016ISI: 000247823000001ScopusID: 2-s2.0-34249659123OAI: diva2:11128
QC 20100827Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2011-11-07Bibliographically approved
In thesis
1. Life prediction and mechanisms for the initiation and growth of short cracks under fretting fatigue loading
Open this publication in new window or tab >>Life prediction and mechanisms for the initiation and growth of short cracks under fretting fatigue loading
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Fretting fatigue is a damage process that may arise in engineering applications where small cyclic relative displacements develop inside contacts leading to detrimental effects on the material fatigue properties. Fretting is located in regions not easily accessible, which makes it a dangerous phenomenon. It is therefore important to be able to make reliable predictions of the fretting fatigue lives.

The work presented in this thesis has its focus on different aspects related to fretting fatigue in the titanium alloy Ti-17. A fretting experiment was developed which allowed for separate control of the three main fretting loads. Initially, the evolution of the coefficient of friction inside the slip region was investigated experimentally and analytically. Subsequently, 28 fretting tests were performed in which large fatigue cracks developed.

The fretting tests were firstly evaluated with respect to fatigue crack initiation through five multiaxial fatigue criteria. The criteria predicted a too high fretting fatigue limit. A possible clue to the discrepancy was found in the fretting induced surface roughness with the asperity-pit interactions.

The fatigue growth of the large fretting cracks was numerically modelled through a parametric crack growth procedure. The predicted lives were compared to the experimental outcome. The numerical simulations showed that linear elastic fracture mechanics was an appropriate tool for the prediction of fretting fatigue propagation lives in the long crack regime.

Fatigue cracks spend most of their propagation life in the small crack regime. The possibility of modelling the small crack behaviour is therefore very important from the engineering point of view. The fatigue growth of through thickness short cracks was studied experimentally and numerically in the four-point bend configuration. It was found that linear elastic fracture mechanics and closure-free material growth data furnished conservative estimates for cracks longer than 50 μm.

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental results on fretting life with or without shot peening were simulated. The fatigue life enhancement in shot peened specimens could be explained by slower crack growth in the surface material layer with residual compressive stresses.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 29 p.
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0423
Fretting fatigue; Fretting experiment; Friction evolution; Fatigue crack initiation; Fatigue crack growth; Short crack; Non-destructive testing; Acoustic emission; Potential drop; Shot peening; Stress relaxation; Crack closure; Finite element method; Titanium alloy.
National Category
Mechanical Engineering
urn:nbn:se:kth:diva-4185 (URN)
Public defence
2006-12-06, F3, Lindstedtsvägen 28, KTH - Stockholm, 10:00

QC 20100827

Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2013-01-14Bibliographically approved

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